An image sensor includes a plurality of pixels, and each of the pixels generates an electrical signal corresponding to an image signal incident on the image sensor. Each of the pixels includes a photodiode that directly receives the image signal. As an area of the photodiode increases, the photodiode can receive the image signal more effectively. According to current trends in technology, a resolution of the electrical signal corresponding to the image signal is increased by increasing the number of pixels included in the image sensor, and a size of the image sensor is decreased. Therefore, technical means for decreasing the area of the image sensor and the area of each of the pixels have been studied.
FIG. 1 is a circuit diagram illustrating a unit pixel of a complementary metal-oxide-semiconductor (CMOS) image sensor.
Referring to FIG. 1, the unit pixel 100 includes a photodiode PD, a pass transistor M1, a reset transistor M2, a conversion transistor M3, a selection transistor M4, and a capacitor C.
The photodiode PD generates a charge corresponding to the incident image signal. The pass transistor M1 performs a switching operation to pass or block the charge generated by the photodiode PD that is connected to a terminal of the pass transistor M1 to the other terminal thereof in response to a pass control signal PASS. A terminal of the capacitor C is connected to a ground voltage GND, and in the other terminal thereof, the charge passing through the pass transistor M1 that performs the switching operation is accumulated. The capacitor C illustrated in FIG. 1 is an electrical modeling of a floating diffusion area that is used as the other terminal of the pass transistor M1. The reset transistor M2 transmits a charge corresponding to a supply voltage VDD to the capacitor C in response to a reset control signal RESET. The conversion transistor M3 primarily determines an initial voltage value V1 that is determined by charges stored in the capacitor C through the reset transistor M2, and thereafter generates a conversion voltage value V2 corresponding to a voltage value that is changed because the charge generated by the photodiode PD affects the initial voltage value V1. The selection transistor M4 in the pixel outputs the initial voltage value V1 and the conversion voltage value V2 in response to a selection signal SEL.
The unit pixel illustrated in FIG. 1 is an example of conventional unit pixels, and operations of the pixel are well-known by those skilled in the art, so that a detailed description of the operations of the pixel is omitted.
A conversion efficiency of the pixel for converting an image signal into an electrical signal may be represented by a fill factor that is a ratio of the area occupied by the photodiode to the area of the pixel. In consideration of semiconductor manufacturing processes, the fill factor generally ranges from 20% to 45%. The pixel illustrated in FIG. 1 uses four transistors M1 to M4, and areas occupied by these four transistors in the pixel may cause a decrease in the area occupied by the photodiode. Recently, in order to reduce the number of MOS transistors, a method of sharing MOS transistors by adjacent pixels is proposed. Nonetheless, an area occupied by remaining MOS transistors is still large, so that an invention of a separate-type unit pixel is introduced (Korean Patent Application No. 10-2005-0030568, filed by LEE, Do-Young on Apr. 12, 2005).
According to the invention, a pass transistor M1 and a photodiode PD from among four MOS transistors, that are used to generate a charge corresponding to an image signal are manufactured in a wafer, and remaining transistors M2 and M4 that are used to convert the charge into a corresponding voltage so as to be output are manufactured in another wafer in order for the two wafers to be implemented as different chips. Thereafter, the chips are electrically connected to each other to be used. Since the pass transistor M1 and the photodiode that are used to generate the charge corresponding to the image signal are manufactured in a single wafer, an area of the photodiode can be increased, and the fill factor can be significantly improved. The details are disclosed in the above-mentioned Korean Patent Application.
FIG. 2 is a sectional view illustrating a semiconductor wafer of the separate-type unit pixel, in which the photodiode and the pass transistor are implemented, according to the invention.
Referring to FIG. 2, the photodiode PD includes a P-type diffusion area P and an N-type diffusion area N that are joined with each other. The pass transistor represented by a dotted circle is operated by the pass control signal PASS applied to a gate region G that is hatched, and a terminal of the pass transistor is a portion of the photodiode PD including the joined P-type diffusion area P and the N-type diffusion area N and the other terminal thereof is a floating diffusion area FD. The pass transistor in the dotted circle and the photodiode (PN junction region), included in a pixel, are surrounded by a shallow trench insulator (STI) structure. Therefore, electrons generated by the pixel to correspond to an incident image signal are prevented from being transited to an adjacent pixel. However, since a depth of the STI structure is not deep, a portion of the electrons generated by the photodiode may be transited to an adjacent pixel.
This phenomenon is called a cross talk. When the cross talk occurs, there is a problem in that when an electrical signal that is obtained by converting the incident image signal is reproduced, the original image signal is not correctly displayed.